Controlling Terahertz and Near‑Infrared Transmission in FeCl₃‑Intercalated Graphene and WS₂ Thin Films for Advanced THz‑TDS Applications

Abstract

Terahertz time‑domain spectroscopy (THz‑TDS) is rapidly becoming a cornerstone technique in non‑destructive evaluation, biomedicine, and secure communications. Yet, the development of robust, room‑temperature components that can generate, detect, filter, and modulate THz radiation remains a critical bottleneck. In this study we report on the optical response of two engineered two‑dimensional (2D) material systems that exhibit tunable transmission across the near‑infrared (NIR) and terahertz (THz) spectra. We fabricated iron trichloride (FeCl₃)‑intercalated graphene (i‑Graphene) on glass, sapphire, and Kapton substrates, and thin tungsten disulfide (WS₂) films derived from liquid‑crystal (LC) dispersions on Kapton and polyethylene terephthalate (PET). By systematically varying dopant concentration, layer number, and substrate type, we demonstrate that both material families can be engineered to deliver high‑transmission, low‑loss performance in the 0.1–2 THz band while simultaneously enhancing NIR transparency. These findings lay the groundwork for next‑generation THz modulators, polarizers, and waveguides that leverage the intrinsic flexibility and tunability of 2D materials.

Introduction

THz‑TDS, driven by femtosecond near‑infrared lasers, has attracted significant attention due to its capacity for broadband, non‑invasive sensing in fields ranging from material science to biomedical diagnostics [1]. Despite its promise, the technology still suffers from a scarcity of efficient, stable, and scalable components. Traditional THz elements—nonlinear crystals, organic semiconductors, and metamaterials—often require cryogenic operation or complex fabrication steps. Two‑dimensional materials, in contrast, offer atomically thin footprints, exceptional carrier mobilities, and a rich spectrum of tunable electronic properties that can be tailored by layer number, chemical doping, and substrate engineering [2]. Graphene and its derivatives, transition‑metal dichalcogenides (TMDs) such as MoS₂, WSe₂, and WS₂, and layered oxides have already been shown to exhibit pronounced THz conductivity, high‑frequency harmonics, and ultrafast carrier dynamics [3]. For THz‑TDS, ambient‑stable materials are paramount; hence, we focus on room‑temperature systems that do not demand cryogenic cooling. Previous works have highlighted graphene as a versatile detector, polarizer, and modulator [4], while WS₂ has been explored as a THz emitter and magnetically tunable modulator [5]. An often overlooked aspect is the influence of the supporting substrate. Transparent substrates such as high‑density polyethylene (HDPE), polytetrafluoroethylene (PTFE), cyclic olefin copolymer (Topas), Kapton, and PET are routinely used in THz optics due to their low absorption across 0.1–2 THz [6]. However, each substrate interacts differently with the overlying 2D layer, altering electronic doping, strain, and interfacial scattering. Understanding these coupled effects is essential when designing devices that combine 2D materials with conventional THz optics. Here we investigate FeCl₃‑intercalated graphene on glass, sapphire, and Kapton, a system that had not previously been studied in the NIR and THz domains. In parallel, we fabricate WS₂ films from LC‑phase dispersions and transfer them to Kapton and PET. By controlling dopant level, layer thickness, and substrate choice, we demonstrate precise modulation of transmission in both spectral regimes, thereby enabling the development of high‑performance THz modulators and other components.

Experimental Methods

Sample Fabrication

Figure 1a shows the schematic of the layered structures examined. Graphene layers (single‑layer graphene—SLG; few‑layer graphene—FLG, 5–6 atomic layers; multilayer graphene—MLG, 50–60 layers) were synthesized via chemical vapour deposition (CVD) on copper or nickel substrates, using methane as the carbon precursor. FLG and MLG were subsequently intercalated with FeCl₃ vapours in a three‑zone furnace [30], producing i‑FLG and i‑MLG. The intercalated sheets were transferred onto glass (1 mm), sapphire (0.8 mm), and Kapton (0.125 mm) by first coating them with polymethylmethacrylate (PMMA), etching the metal catalyst with concentrated ferric chloride, and finally dissolving the PMMA in acetone. The intercalated films have been extensively characterized in previous work, confirming high crystalline quality and uniform FeCl₃ distribution [41]. WS₂ films were prepared from LC‑phase dispersions. Bulk WS₂ powder (Sigma‑Aldrich 243639, ~µm‑size) was dispersed in isopropanol (IPA) at 5 mg mL⁻¹, followed by 5 h of ultrasonic exfoliation with intermittent 30 min breaks to prevent overheating. A 2000 rpm, 10 min centrifugation step removed unexfoliated particles. The supernatant was vacuum‑dry‑re‑dispersed in IPA at 1, 5, and 100 mg mL⁻¹. The final concentrations were verified by optical density measurements. Drop‑casting (50 µL) onto Kapton yielded two films: WS₂ S (low‑concentration, non‑LC fraction) and WS₂ L (high‑concentration, LC fraction). For PET, a thin‑film transfer method was employed: 20 mL of LC dispersion was filtered onto a PTFE membrane, then the membrane was wetted with IPA and heated to 70 °C, allowing the WS₂ film to detach and adhere to the PET substrate. Two PET samples were produced: WS₂_LC (1 mg mL⁻¹) and WS₂_HC (5 mg mL⁻¹). SEM and AFM imaging confirmed lateral particle sizes of ~2.5 µm² and thickness of ~3.9 nm; overall film thicknesses ranged from 1 to 10 µm.

Raman Spectroscopy

Raman spectra were recorded using a Renishaw spectrometer with 532 nm excitation (0.1 mW) and 10 s integration. This technique enabled us to assess layer number, doping, and defect density across all samples.

Visible and IR Spectroscopy

Transmittance in the 190–1100 nm range was measured with a Evolution‑300 spectrophotometer, achieving <0.05 nm wavelength precision and 1 % photometric accuracy.

Terahertz Spectroscopy

We employed a home‑built THz‑TDS system based on optical rectification in an InAs crystal within a 2.4 T magnetic field. A Yb‑doped femtosecond oscillator (1050 nm, 100 fs, 70 MHz) split the beam into pump and probe paths; the pump generated THz pulses (30 µW average, ~1.8 ps FWHM) that impinged on the sample. The transmitted THz pulse was electro‑optically sampled in a CdTe crystal using a balanced photodetector and lock‑in amplification. Data were acquired at multiple points per sample and averaged; a 3 mm beam diameter ensured uniform illumination. Fourier transformation of the time‑domain traces yielded frequency‑domain transmittance spectra from 0.2 to 1 THz.

Results and Discussion

Raman analysis (Fig. 2) confirmed the high crystalline quality of all graphene variants; the G peak resided between 1582–1591 cm⁻¹, while the 2D peak displayed the expected layer‑dependent shifts. Intercalated graphene exhibited a distinct G‑peak splitting (G₀, G₁, G₂) corresponding to random, single‑layer, and bilayer FeCl₃ doping states, respectively [30]. No significant D‑band intensity indicated negligible defect introduction during intercalation. Visible‑NIR transmittance (Fig. 4) revealed that FeCl₃ intercalation enhances transparency in the 700–1100 nm window by up to 10 % at 1000 nm, attributable to Pauli blocking from carrier filling [54]. WS₂ films displayed a thickness‑dependent transmittance drop of up to 35 % across 400–1100 nm, reflecting increased optical density with higher LC concentration. THz transmittance (Fig. 5) showed a linear decline with graphene layer count for all substrates, confirming that the absorption coefficient remains constant in the 0.1–1 THz band. FeCl₃ intercalation modified the THz response differently depending on the substrate: on Kapton, transmission increased by ~30 % in the 0.4–0.8 THz range, whereas sapphire exhibited a comparable absorption rise. These substrate‑dependent scattering effects highlight the importance of interfacial engineering for device optimization. WS₂ films maintained high THz transparency (Fig. 6); by selecting LC concentration, the transmission could be tuned without compromising NIR performance. Moreover, the magnetic tunability of LC‑phase WS₂ suggests potential for magnetically driven THz modulators, analogous to spin‑current THz oscillators [57].

Conclusions

We have demonstrated that FeCl₃‑intercalated graphene and LC‑derived WS₂ films can be engineered to deliver controllable transmission in both the NIR and THz ranges. By judiciously selecting dopant levels, layer thickness, and substrate, these 2D systems can serve as the building blocks for high‑performance THz modulators, polarizers, and waveguides. Future work will focus on integrating these materials into prototype THz‑TDS devices and exploring magnetic field modulation of WS₂ for active THz control.

Availability of Data and Materials

The datasets generated and analyzed in this study are available from the corresponding author upon reasonable request.

Abbreviations

AFM

Atomic force microscopy

CVD

Chemical vapour deposition

EO

Electro‑optical

FLG

Few‑layer graphene

i‑FLG

Intercalated few‑layer graphene

i‑MLG

Intercalated multilayer graphene

i‑SLG

Intercalated single‑layer graphene

IPA

Isopropanol

LC

Liquid crystal

MLG

Multilayer graphene

PET

Polyethylene terephthalate

PMMA

Polymethyl methacrylate

SEM

Scanning electron microscopy

SLG

Single‑layer graphene

THz‑TDS

Terahertz time‑domain spectroscopy